In recent years, silver nanoparticles have been applied in various industrial fields, because they have various effects, including sterilizing and deodorizing effects. However, there have been frequent reports that silver nanoparticles are absorbed into the human body through skin tissue or a respiratory organ to give harm to the human body. Thus, problems associated with the safety of the use of silver nanomaterials have been frequently discussed.
In an attempt to overcome the above problems by increasing the dispersibility of silver nanoparticles, methods of adsorbing or binding silver nanoparticles to other materials have been reported. For example, Korean Patent Registration No. 764535 discloses a dispersion of metal nanoparticles and a preparation method thereof. However, materials which are used in the above patent are mostly organic compounds such as dodecylamine or oleic acid, which are subjected to a reduction reaction in a subsequent process. In this reduction reaction, metal powder is separated from the organic compounds, and the binding strength between the metal powder and the organic compounds is significantly reduced. Due to such problems, there are limitations in ensuring the stability of silver nanoparticles.
In addition, Korean Patent Registration No. 10-0806915 discloses a method for binding silver nanoparticles to silica. In this method, silica powder is dispersed together with sufficient amounts of a silver ion precursor and a stabilizing agent at a temperature of about 100 □ and an alkaline pH, and a reducing agent is added thereto while controlling stirring speed and stirring conditions. This method has problems in that the preparation process is complex, high temperature and basic conditions are required and large amounts of various compounds should be added. Silver particles bound by this method have a large particle size and adhere to the outer surface of silica so that they are likely to be detached from silica. Thus, this method has limitations in ensuring the stability of silver nanoparticles.
In addition, Korean Patent Laid-Open Publication No. 10-2007-006887 discloses a method of preparing reverse micelle nanoparticles using an organic solvent. As shown in FIG. 1a, magnetite (Fe3O4) micelles of inorganic nanoparticles which can be included in silica nanoparticles are formed in an organic solvent. Then, as shown in FIG. 1b, the magnetite (Fe3O4) micelles of inorganic nanoparticles are self-assembled into reverse micelles by a sol-gel reaction with a hydrophilic group in an aqueous solution containing a surfactant (such as oleic acid) dispersed therein, thereby making magnetite (Fe3O4) nanoparticles (Nat. Materials, 2004, 3, 891-895). Then, as shown in FIG. 1c, the nanoparticles are allowed to react with a surfactant as a template in an aqueous micelle solution, thereby forming mesoporous silica. This method has problems in that the preparation process is complex and is uneconomical, because expensive inorganic nanoparticles are included in mesoporous silica nanoparticles. Also, the process of making mesoporous silica nanoparticles is not environmentally friendly, because it is carried out using a volatile organic solvent (chloroform) and ethyl acetate under basic conditions caused by ammonia. In addition, this method is cost-ineffective, because two kinds of surfactants (oleic acid and CTAB) are used.
In a paper relating to a method for preparing spherical mesoporous silica containing nanosized silver (Hae-Joon Park et al., “New synthetic product of silica containing nanosized silver for inhibiting various plant diseases”, Plant Pathol. J. 22(3), 295-302, 2006), silver-containing silica is prepared by adding a soluble polymer, silver nitrate and sodium silicate (Na2SiO3) to distilled water. In this paper, as shown in FIG. 1(a), PVP, sodium silicate and silver nitrate are dissolved in IPA by irradiation of 25 kGy of gamma rays and subjected to a sol-gel reaction in a nitrogen atmosphere, and as a result, three kinds of nanoparticle materials are randomly present and nanoparticles are actually prepared as shown in FIG. 2(b). However, it can be seen that silica and silver nanostructures are not complete structures. In other words, the binding between silica structures and silver nanoparticles is loose and silver nanoparticles are non-uniform and likely to be separated. In addition, in a paper (Jae-Hyun Park et al., “Preparation of silver/mesoporous silica by direct reaction of silver and silver/functional mesoporous silica by silver-adsorbed structures”, Materials Letters 61, 156-159, 2007), a method for introducing silver nanoparticles into a mesoporous material is described. In this method, the surface of a mesoporous material is treated with mercaptopropyl silane which is a functional group, and particles are bound to the surface, followed by synthesis. In this method, the production cost is high, the synthesis process is difficult to carry out, and the uniformity of particles is problematic.